DE102020212701A1 - Process to reduce pores and to increase the penetration depth when welding copper materials - Google Patents

Abstract

The invention relates to a method for reducing pores (26) and increasing the welding depth (58, 62) in a workpiece (42) when welding copper materials using laser beam welding. At least the following method steps are carried out: Active oxidation (24) of a surface (44) of a workpiece (42) takes place by a single pass (58) of a laser beam (40) with reduced laser power along a weld seam geometry (36), carrying out at least one second pass (60) along the weld seam geometry (36), pretreated according to a) with reduced laser power according to a) using increased absorption of the laser beam (40) by the modification of the surface (44) caused in a) or alternatively a feed takes place at least one second pass (60) of the laser beam (40) with laser power > reduced laser power according to a) along the weld seam geometry (36) using increased absorption of the laser beam (40) due to the modification of the surface (44) caused in a) to increase a first welding depth (56) to at least a second welding depth (62) in the workpiece (42).Darü The invention also relates to the use of the method for welding Cu-coated substrates to plastic or ceramics with a Cu metallization thickness of less than 100 μm and to welding foil stacks made of copper foils in battery production or electronics production.

Description

technical field

The invention relates to a method for reducing pores and increasing the welding depth in a workpiece when welding copper materials using laser beam welding. Furthermore, the invention relates to the use of the method for welding Cu-coated substrates made of plastic or ceramics with very thin copper metallization layers and to the use of the method for use in battery production and electronics production.

State of the art

The increasing electrification of public and private passenger transport (electromobility) places increased demands on the manufacturing and joining technology of individual components of electrical assemblies. The need for high current-carrying capacities in combination with sufficient service life requires the use of copper materials in many cases. It must be possible to connect these materials to one another in an electrically conductive manner with a view to achieving the highest possible joining quality and high productivity. It is often important to ensure a very defined energy input in combination with narrow quality tolerances.

Laser beam welding has already established itself as a highly flexible joining process in industrial manufacturing technology, but it poses numerous challenges when welding copper materials. The low absorption of laser radiation from typical welding lasers with a wavelength of around 1 µm requires very high intensities on the component in order to be able to carry out a welding process at all. In addition, the high thermal conductivity of copper makes the heat input considerably more difficult. When the melting temperature is reached, the absorption suddenly increases by a factor of 2 to 3, while the thermal conductivity drops by a factor of about 2. This leads to a sharp increase in the energy input, combined with the sudden formation of a vapor channel (keyHhole) with a large aspect ratio (ratio of keyhole depth to keyhole diameter).

On the one hand, such small welding depths (100 µm to 500 µm) can only be achieved through the combination of laser radiation that can be focused very well (good beam quality) and a suitable process strategy (e.g. local beam oscillation). Further challenges result from the keyhole instabilities that occur, which are unavoidable with high aspect ratios and can lead to spatter or melt ejections.

Presentation of the invention

1. a) Aktive Oxidation einer Oberfläche eines Werkstücks durch einmalige Überfahrt eines Laserstrahls mit reduzierter Laserleistung entlang einer Schweißnahtgeometrie,
2. b) Durchführen mindestens einer zweiten Überfahrt entlang der Schweißnahtgeometrie, vorbehandelt gemäß a) mit reduzierter Leistung gemäß a) und Ausnutzung einer erhöhten Absorption des Laserstrahls durch die in a) hervorgerufene Modifikation der Oberfläche, oder
3. c) Durchführen mindestens einer zweiten Überfahrt des Laserstrahls mit Laserleistung > reduzierter Laserleistung gemäß a) entlang der Schweißnahtgeometrie unter Ausnutzung einer erhöhten Absorption des Laserstrahls durch die in a) hervorgerufene Modifikation der Oberfläche zur
4. d) Erhöhung einer ersten Einschweißtiefe auf mindestens eine zweite Einschweißtiefe im Werkstück.

According to the invention, when welding copper materials by means of laser beam welding, a method for reducing pores and increasing the welding depth in a workpiece is proposed, with at least the following method steps being carried out:

1. a) Active oxidation of a workpiece surface by a single pass of a laser beam with reduced laser power along a weld seam geometry,
2. b) Carrying out at least one second pass along the weld seam geometry, pretreated according to a) with reduced power according to a) and utilizing increased absorption of the laser beam due to the surface modification caused in a), or
3. c) Carrying out at least one second pass of the laser beam with laser power> reduced laser power according to a) along the weld seam geometry using increased absorption of the laser beam due to the modification of the surface caused in a).
4. d) increasing a first welding depth to at least a second welding depth in the workpiece.

Active oxidation can be carried out with the method proposed according to the invention, which significantly reduces the presence of hydrogen, in particular from the environment of the welding process, which in turn significantly improves the mechanical strength of the weld seam obtained when the welding operation is carried out. Furthermore, with the method proposed according to the invention, the formation of an oxide layer, which is produced during a first pass over the weld seam geometry, can be used to further penetrate the melted structure, i. H. to achieve a significant increase in the welding depth.

In an advantageous development of the method proposed according to the invention, a first welding depth is increased to at least a second welding depth in the workpiece through the combination of a) and b) compared to carrying out b) exclusively.

Furthermore, a first welding depth is advantageously increased at least a second welding depth in the workpiece through the combination of a) and c) compared to carrying out c) exclusively.

In an advantageous further development of the idea on which the invention is based, H ₂ uptake is reduced by a combination of a) and b) compared to carrying out b) alone. Furthermore, in the method proposed according to the invention, a reduction in H ₂ uptake can be achieved by a combination of a) and c) compared to c) alone.

In an advantageous development of the method proposed according to the invention, laser beam welding is carried out with laser radiation below a wavelength of 0.6 μm, preferably with a wavelength of green laser light of 0.515 μm and more preferably with a wavelength of blue laser light in the range from 0.44 μm to 0. 46 µm performed.

In the method proposed according to the invention, the temperature τ on the surface of the workpiece is below the evaporation temperature of the copper while method step a) is being carried out.

Furthermore, in the method proposed according to the invention, the degree of absorption of laser radiation in copper oxide exceeds the degree of absorption of laser radiation in bare copper many times over.

In the method proposed according to the invention, welding depths in the range between 20 μm and 300 μm can be achieved with a combination of method steps a) and b).

In one embodiment of the method proposed according to the invention, a combination of method steps a) and c) can lead to welding depths in the range between 100 μm and 2000 μm.

In the method proposed according to the invention, the number of passes along the weld seam geometry when carrying out method steps a) and b) is <10, preferably <5.

In the method proposed according to the invention, the number of passes along the weld seam geometry when carrying out method steps a) and c) can alternatively be <10, preferably <5.

Carrying out the method proposed according to the invention advantageously enables the production of the oxide layer on the surface of the workpiece by the laser beam itself. Further processing steps for producing the oxide layer on the surface of the workpiece can therefore be omitted.

In addition, the invention relates to the use of the method for welding Cu-coated substrates made of plastic or ceramics, in which the Cu metallization thickness is between 20 μm and 500 μm. In addition, the invention relates to the use of the method for welding foil stacks made of copper foils in the field of battery production. Furthermore, the invention relates to the use of the method in electronics production for joining thinly coated flex foils with a thickness of between 10 μm and 100 μm on thinly coated substrates with a thickness of between 10 mm and 100 mm.

Advantages of the Invention

According to one aspect of the present invention, the reduction in porosity caused by the presence of H ₂ during laser beam welding of copper materials is prevented by applying an oxide layer to the workpiece surface. The copper oxide on the surface of the workpiece to be welded by means of laser welding leads to a binding of the ambient hydrogen on the surface. In the process, water vapor is formed, which can escape from the weld zone near the surface without porosity occurring in the weld seam volume.

A corresponding oxidation of the workpiece surface can take place in advance on the workpiece by means of heat treatment, in particular by the laser beam itself before the actual welding process is carried out. For this purpose, the laser beam runs over the weld seam geometry with a reduced laser power, which results in the heat treatment.

It is important to ensure that the temperature at the tip of the laser beam, which essentially corresponds to the temperature at the surface of the material, does not exceed the vaporization temperature. This procedure, also known as heat conduction welding, can minimize the heat input into the workpiece and produce a sufficiently thick oxide layer. In a further process step, the actual welding process takes place along a weld path geometry that is identical to that of the heat treatment. With the solution proposed according to the invention, the defect of an impermissibly high formation of hydrogen pores in the weld seam can be significantly reduced and ideally completely eliminated. Through the oxidation the surface of the workpiece, in particular the use of expensive protective gases, such as helium, argon and nitrogen, and the formation of a protective gas atmosphere can be dispensed with, and the associated costs can be saved. The use of additional material, such as titanium, silicon or aluminum, can also be omitted and production costs can be reduced accordingly.

If the copper materials are joined together using green laser radiation, a very stable welding process can be achieved which, in particular, guarantees a constant welding depth and, on the other hand, can be carried out with little or no spatter. By avoiding the hydrogen-induced porosity, the advantages of this process can be transferred without further consideration of a possible strength-reducing influence of pore formation. In particular, weld seams can be produced on substrates made of plastic or ceramic, which have copper metallizations of a few hundred micrometers, without the substrates made of ceramic or plastic being damaged by the laser beam.

According to a further aspect of the method proposed according to the invention, a greater welding depth can be achieved by utilizing heat accumulation effects caused by multiple passes over the workpiece by the laser beam. If, according to the method proposed according to the invention, the weld seam geometry is passed over several times, an actively generated oxide layer can be produced on the upper side of the weld seam due to the associated oxide formation during welding. This has two advantages: During a second pass with the laser beam, which is operated at the nominal laser power, the energy of the laser beam is absorbed into the material via the oxide on the surface at the point to be welded. The degree of absorption of copper oxide is several times higher than the degree of absorption of common laser wavelengths in bare copper. As a result, more energy is deposited in the workpiece and the welding depth increases. Secondly, the formation of copper oxide takes place with the release of energy as part of an exothermic reaction, which in turn leads to increased deposition of energy in the workpiece and thus to an increased welding depth.

The method proposed according to the invention significantly increases the welding depth in the case of laser beam welding to be carried out without spatter. This brings decisive advantages for many potential welding processes, especially in electronics and battery production. If, for example, foil stacks made of copper foil are welded in battery production, a thicker foil stack can be welded reliably and with high quality by using the method proposed according to the invention. This is due on the one hand to the essentially non-porous weld seam and on the other hand to the increase in the welding depth that can be achieved with the method proposed according to the invention. In electronics manufacturing, thin foils (flex foils) are often attached to thinly coated substrates. With the conventional heat conduction welding process, the thickness of the flex foils that can be safely welded is limited to a few tens of micrometers. By using the method proposed according to the invention, these limits can be shifted upwards.

Another striking advantage of the method proposed according to the invention can be seen in the fact that the oxide on the workpiece surface (seam upper bead) is removed by the tool itself, i. H. generated by the laser beam. For reliable coupling of the energy of the laser beam, it is not unusual according to the prior art to coat the surface of the workpiece with tin or nickel, for example. This pre-treatment can be avoided by the method proposed according to the invention with induced oxide layers on the surface, as a result of which pre-treatment work steps can be saved when carrying out automated welding processes.

character list

Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

• 1 den schematischen Zusammenhang zwischen Wasserstofflöslichkeit und Temperatur für Cu,
• 2 gegenübergestellte Querschnitte lasergestrahlter Proben aus Reinkupfer mit aktiv oxidierter Oberfläche beziehungsweise im unbehandelten Zustand,
• 3 lichtmikroskopische Aufnahmen laserstrahlgeschweißter Kupferproben in Dunkelfeld- und Hellfelddarstellung,
• 4.1, 4.2 schematische Darstellung der ersten Überfahrt sowie der zweiten Überfahrt, wobei der Laserstrahl mit Laser-Nennleistung betrieben wird,
• 5 den Verlauf der erreichbaren Einschweißtiefe laserstrahlgeschweißter Kupferproben aufgetragen über die Anzahl der Überfahrten.

Show it:

• 1 the schematic relationship between hydrogen solubility and temperature for Cu,
• 2 compared cross-sections of laser-blasted samples made of pure copper with an actively oxidized surface or in the untreated state,
• 3 light micrographs of laser beam welded copper samples in dark field and bright field representation,
• 4.1 , 4.2 schematic representation of the first pass and the second pass, with the laser beam being operated at the rated laser power,
• 5 the course of the achievable welding depth of laser beam welded copper samples plotted against the number of passes.

Embodiments of the invention

In the following description of the embodiments of the invention, the same or similar elements are denoted by the same reference symbols, with a repeated description of these elements being dispensed with in individual cases. The figures represent the subject matter of the invention only schematically.

According to the illustration 1 shows the connection between the hydrogen solubility and the temperature for the material copper. According to 1 the pore formation in the copper material is due to the fact that copper in the liquid state has a high hydrogen solubility. In 1 the course of a hydrogen solubility 10 is plotted over the temperature course 12. Position 14 designates a melting temperature of the copper material. The hydrogen absorption of the copper is favored by the high diffusion rates of the hydrogen in liquid copper. If the melting temperature 14 is reached during cooling, the hydrogen solubility suddenly drops by a factor of about 3 and the hydrogen-rich melt releases the hydrogen into the environment as a gas. This leads to the porosity mentioned several times above, as in 2 indicated by position 26.

2 shows a comparison of cross sections of laser beam welded samples of pure copper with constant welding parameters. While in the left part of 2 an oxidized surface 18 by performing an active oxidation 24 is that in the right part of FIG 2 surface shown is a bare, untreated surface 20 of copper. Laser beam welding is carried out on both samples (cf. item 22). Since no hydrogen is present in the laser beam weld 22 pretreated with active oxidation 24, neither from the environment nor from contamination on the surface, the relatively uniform laser beam weld 22 is established. Position 28 designates an area in which there is a structural change in the copper material in the workpiece.

If, on the other hand, there is a bare, untreated surface 20 in the area of the laser beam weld 22, pores 26 are formed, which are formed in the material of the weld seam after the laser beam weld 22. The pores 26, which are present in the area 28 with structural changes, weaken the achievable mechanical strength of the laser beam weld 22 not inconsiderably.

According to the illustration 3 microsections can be taken that show light microscopic images of laser-beam welded copper samples. Active oxidation 24 of the surface 44 of a workpiece 42 can be detected in the micrograph, since the copper oxides 32 dissolved in the weld metal appear red under polarized light and in the dark field 30 . The 4.1 and 4.2 Schematic representations of passages over a weld seam geometry 36 can be found.

After the pretreatment of the surface 44 has been carried out, ie its active oxidation 24, for example by a single heat treatment of a surface 44 of a workpiece 42 using a laser beam 40 with reduced laser power, according to FIG 4.1 in the feed direction 48 the feed of the laser beam 40, which is operated at nominal power. A beam tip 46 of the laser beam 40 travels according to the feed speed of the laser beam 40 in the feed direction 48 along the weld seam geometry 36. During a first pass 58 in the feed direction 48, an oxide layer 50 is produced on the surface 44 of the workpiece 42. This covers a seam upper bead 54 of a weld seam 52. When the laser beam 40 first passes over 58 along the weld seam geometry 36, a first welding depth 56 is reached. The formation of the oxide layer 50 during welding and its presence on the upper side of the weld seam 52, ie on the seam upper bead 54 has the following advantages:

At an in 4.2 illustrated second crossing 60 of the laser beam 40 in the feed direction 48 at feed speed over the oxide layer 50 applied during the first crossing 58, the energy of the laser beam 40 is more strongly absorbed via the oxide layer 50 on the surface 44 in the copper material. The degree of absorption of the oxide layer 50 made of copper oxide is higher by a factor than the degree of absorption of common laser wavelengths in copper material. Consequently, more energy is deposited in the workpiece 42 and the weld depth increases from the weld depth 56 generated during the first pass 58 to the second weld depth 62 generated by the second pass 60. It should also be noted that the formation of copper oxide, ie the formation of the oxide layer 50 takes place with the release of energy as an exothermic reaction, which in turn leads to an increased deposition of energy in the workpiece 42 and thus enables the second increased welding depth 62 .

In the representation according to 5 is the course of an achievable welding depth for laser beam welded copper samples plotted against the number of passes. A progression of the welding depth is indicated by reference number 70, plotted over a number 72 of passes according to the in the 4.1 and 4.2 indicated weld seam geometry 36.

The respective crossings along the weld seam geometry 36 were carried out with a first parameter set 74, for example. Corresponding to the first parameter set 74, a laser beam 40 was operated with a first power P ₁ and a first feed rate. Corresponding to the passes, which were less than 5, welding depths of up to 110 µm are achieved. If the feed rate is increased, ie the feed rate v ₂ exceeds the feed rate v ₁ - with constant laser power P ₁ - welding depths of up to 80 µm are obtained with up to 10 passes. If the feed rate is increased again at constant laser power, ie v ₃ > v ₂ > v ₁ , then the welding depth is reduced from less than 20 µm to up to 60 µm when passing over.

With an increase in laser power to P ₂ and an increased feed rate of z. B. 1 m/s, on the other hand, welding depths of up to 85 µm are achieved with two passes. If the feed speed of the laser beam 40 in the feed direction 48 is doubled in comparison to the fourth parameter set 80 at a constant laser power P ₂ , only welding depths of the order of magnitude between 30 and 40 μm are achieved.

This means that the highest welding depths are achieved at a low feed rate in the feed direction 48 . According to the illustration 5 shows that, regardless of the parameters of laser power and welding feed, the greatest increase in welding depth is recorded during the second pass. This is directly attributable to the surface 44 of the workpiece 42 being oxidized during the first previous pass.

The invention is not limited to the exemplary embodiments described here and the aspects highlighted therein. Rather, within the range specified by the claims, a large number of modifications are possible, which are within the scope of expert action.

Claims (16)

Method for reducing pores (26) and for increasing the welding depth (56, 62) in a workpiece (42) when welding copper materials using laser beam welding, with at least the following method steps: a) Active oxidation (24) of a surface (44) of a workpiece (42) by a single pass (58) of a laser beam (40) with reduced laser power along a weld seam geometry (36), b) carrying out at least one second pass (60) along the weld seam geometry (36), pretreated according to a) with reduced laser power according to a) below utilizing increased absorption of the laser beam (40) by the modification of the surface (44) brought about in a), or c) Carrying out at least one second pass (60) of the laser beam (40) with laser power > reduced laser power according to a), along the weld seam geometry (36) using increased absorption of the laser beam (40) due to the modification of the surface (4) caused in a) 44) to d) increasing a first welding depth (56) to at least a second welding depth (62) in the workpiece (42). procedure according to claim 1 , characterized in that an increase in a first welding depth (56) to at least a second welding depth (62) in the workpiece (42) takes place through the combination of a) and b) compared to the exclusive implementation of b). procedure according to claim 1 , characterized in that an increase in a first welding depth (56) to at least a second welding depth (62) in the workpiece (42) takes place through the combination of a) and c) compared to carrying out c) exclusively. procedure according to claim 1 , characterized in that a reduction in H ₂ uptake occurs through a combination of a) and b) compared to carrying out b) alone. procedure according to claim 1 , characterized in that a reduction in H ₂ uptake occurs through a combination of a) and c) compared to c) alone. procedure according to claim 1 , characterized in that the laser beam welding is carried out with laser radiation below a wavelength of 0.6 µm, preferably with a green laser light wavelength of 0.515 µm and more preferably with a blue laser light wavelength in the range from 0.44 to 0.46 µm. procedure according to claim 1 , characterized in that during the implementation of method step a) the temperature τ on the surface (44) of the workpiece (42) is below the vaporization temperature of copper. Method according to one of Claims 1 until 7 , characterized in that the degree of absorption of laser radiation in copper oxide the absorp level of laser radiation in bare copper many times over. Method according to one of the preceding claims, characterized in that with a combination of method steps a) and b) welding depths (56, 62) in the range between 20 µm and 300 µm can be achieved. Method according to one of the preceding claims, characterized in that with a combination of method steps a) and c) welding depths (56, 62) in the range between 100 µm and 2000 µm can be achieved. Method according to one of the preceding claims, characterized in that the number of passes (58, 60) along the weld seam geometry (36) when carrying out method steps a) and b) is <10, preferably <5. Method according to one of the preceding claims, characterized in that the number of passes (58, 60) along the weld seam geometry (36) when carrying out method steps a) and c) is <10, preferably <5. Procedure according to claims 1 until 11 , characterized in that the oxide layer (50) on the surface (44) of the workpiece (42) by the laser beam (40) itself is generated. Use of the method according to any one of the preceding claims for welding Cu-coated substrates made of plastic or ceramic, in which there is a Cu metallization thickness of between 20 µm and 500 µm. Use of the method according to any one of Claims 1 until 12 for welding foil stacks made of copper foils in battery production. Use of the method according to any one of Claims 1 until 12 in electronics manufacturing for joining thinly coated flex foils with a thickness of 10 µm to 100 µm on thinly coated substrates with a thickness of 10 µm to 100 µm.

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